1. Field of the Invention
The invention concerns repositionable pressure-sensitive adhesive tapes or sheets such as are used in making road signs and for application to vehicles for purpose of information and/or decoration.
2. Description of the Related Art
U.S Pat. Nos. 3,331,729 and 3,413,168 (Danielson et al) describe that a large pressure-sensitive adhesive tape or sheet can be made repositionable by partially embedding into the adhesive layer a large number of tiny hollow microspheres, there called "microballoons". The Danielson tape is made by randomly attracting microballoons to a polyethylene-coated paper liner, partially pressing the microballoons into the polyethylene while it is being softened by heat, applying a pressure-sensitive adhesive over the microballoons, and covering the exposed surface of the adhesive with a decorative film. Then after stripping off the paper liner, the protruding microballoons permit the adhesive-bearing decorative film to be slid along the surface of a substrate until it is precisely positioned, whereupon hand pressure is applied to crush the microballoons, thus allowing the adhesive to contact and to become bonded to the substrate.
Large-scale manufacture of the Danielson article requires considerable skill to ensure that the microballoons are uniformly distributed in order to keep every part of the pressure-sensitive adhesive layer from contacting and sticking prematurely to the substrate. Manufacture also requires a special release liner into which the microballoons can be partially embedded.
U.S Pat. No. 3,314,838 (Erwin) describes a repositionable pressure-sensitive adhesive sheet which is similar to those of the Danielson patents except being made by dispersing microballoons into a spreadable liquid from which the pressure-sensitive adhesive is to be coated. The microballoons "show a tendency to appear at the exposed surface" of the resulting pressure-sensitive adhesive layer, thus giving it a pebbled surface that is said to make the sheet repositionable until the microballoons are crushed (col. 2, lines 56-68).
U.S. Pat. No. 4,376,151 (Parrotta) shows a pressure-sensitive adhesive sheet which, like that of Erwin, has hollow microspheres or microballoons at the surface of its pressure-sensitive adhesive layer, but its adhesive has substantially no adhesive tack except upon application of a threshold pressure. The Parrotta sheet is said to differ from that of Erwin because the latter is activated by very low pressure and is somewhat tacky to the fingers, thus making it unsuitable for the business-form use for which Parrotta sheet is intended.
U.S. Pat. No. 3,301,741 (Henrickson et al.) makes a pressure-sensitive adhesive sheet repositionable by shaping its surface into a pebbly contour of small protrusions that are covered by non-adhesive, continuous, fragile protective caps. Those caps had been formed by embossing polyethylene-coated paper to form a uniform array of depressions, filling the depressions with non-adhesive material such as a solution of polymethylmethacrylate, and wiping the solution off the areas between the depressions. After the sheet has been positioned, pressure is applied, shattering the caps and causing the adhesive to flow around the fragments into intimate contact with a substrate to which the sheet is being applied.
U.S. Pat. No. 4,556,595 (Ochi) makes a pressure-sensitive adhesive sheet repositionable by the random application of tiny solid particles over the adhesive surface. After the sheet has been positioned over a substrate, pressure is applied to force the particles into the adhesive, thus allowing the adhesive to contact and become bonded to the substrate. The Ochi patent preferably employs a "release paper in order to prevent intrusion and dispersion of the non-adhesive solid particles in the adhesive which may occur by application of some pressure during storage" (col. 9, lines 2-7). However, it does not explain how that release paper can prevent the particles from being pushed into the adhesive layer to destroy their utility. Neither does it mention that even if protected from pressure, the particles might migrate into the adhesive during prolonged storage.
In U.S. Pat. No. 3,554,835 (Morgan), the face of a pressure-sensitive adhesive sheet is provided with dots of release material that permit the sheet to be slid over a substrate until pressure is applied to force the adhesive into contact with the substrate.
In U.S. Pat. No. 4,054,697 (Reed et al.), the face of a pressure-sensitive adhesive sheet is provided with a coating of a discontinuous layer of resilient, non-adhesive, solid particles that permit repositioning on a substrate until the particles are deformed under pressure to such an extent as to bring the adhesive and the substrate into fuller contact.
Common to the background discussed above is that when particles are used to make a pressure-sensitive adhesive sheet repositionable, they have always been randomly distributed. Hence, if sufficient particles have been employed to ensure that every portion of the pressure-sensitive adhesive layer is safely kept out of contact with the substrate to which it is being applied, the particles may have such high density as to interfere with the bonding strength. In other words, if high bonding strength is required, it may be necessary to reduce the amount of particles even though some pieces of the pressure-sensitive adhesive sheet might become prematurely bonded to a substrate and thus need to be destroyed.
When the repositionable sheet has a decorative backing such as a thin vinyl film, it can involve the problem that after pressure has been applied to force its pressure-sensitive adhesive into bonding contact with a substrate, the particles or particle fragments that are forced into the adhesive layer can produce a disfiguring pimpling of the decorative backing.